An oleo-pneumatic device or damper absorbs shock through a combination of forcing oil through an orifice and compressing air or another gas. Magnetorheological (MR) fluid is a type of smart fluid that includes microscopic iron particles suspended in a carrier fluid, usually a type of oil. When subjected to a magnetic field, the fluid greatly increases its apparent viscosity, to the point of becoming a viscoelastic solid.
Dampers using MR valves were originally developed to be oleo systems (as opposed to oleo-pneumatic systems), with automobile shock absorbers being a key application. A MR valve comprises an electromagnet that is installed inside of a casing. The casing is usually made from a magnetic alloy, and forms an orifice or annulus within the damper. The MR fluid flows through the orifice or annulus as the damper is compressed or extended. The electromagnet generates a magnetic field that the casing directs perpendicularly through the fluid to an outer casing, which is also typically constructed out of a magnetic alloy. The strength of the magnetic field may be controlled to modify the behavior of the fluid, and therefore the damper, as needed for a particular application or operating environment. Most MR valves use an orifice/annulus that is fixed in size, with the electromagnet located internal to the flow of fluid. A typical MR valve design is therefore not practical to retrofit into existing aircraft landing gear, because of the presence of metering pins in most oleo-pneumatic landing struts. Further, the use of a traditional MR valve would limit the design space for a new MR landing gear design, particularly if it were to be oleo-pneumatic.